ANSYS FLUENT 12.0 UDF Manual - 2.5.14 DEFINE_DPM

You can use DEFINE_DPM_TIMESTEP to change the time step for DPM particle tracking based on user-specified inputs. The time step can be prescribed for special applications where a certain time step is needed. It can also be limited to values that are required to validate physical models.

Argument Type	Description
`symbol name`	UDF name.
`Tracked_Particle *p`	Pointer to the `Tracked_Particle` data structure which
	contains data related to the particle being tracked.
`real ts`	Time step.

Function returns
`real`

There are three arguments to DEFINE_DPM_TIMESTEP: name, p, and ts. You supply the name of your user-defined function. p and ts are variables that are passed by the ANSYS FLUENT solver to your UDF. Your function will return the real value of the DPM particle timestep to the solver.

The following compiled UDF named limit_to_e_minus_four sets the time step to a maximum value of

. If the time step computed by ANSYS FLUENT (and passed as an argument) is smaller than

, then ANSYS FLUENT's time step is returned.

/* Time step control UDF for DPM  */

#include "udf.h"
#include "dpm.h"

DEFINE_DPM_TIMESTEP(limit_to_e_minus_four,p,dt)
{
   if (dt > 1.e-4)
     {
/*       p->next_time_step = 1.e-4; */
       return 1.e-4;
     }

   return dt;
}

The following compiled UDF named limit_to_fifth_of_prt computes the particle relaxation time based on the formula:

$\tau_p = frac{\rho_p d_p^2}{18 \mu} \frac{24}{C_D Re_p}$

(2.5-2)

$Re_p = \frac{\rho d_p \Vert u - u_p\Vert}{\mu}$

(2.5-3)

The particle time step is limited to a fifth of the particle relaxation time. If the particle time step computed by ANSYS FLUENT (and passed as an argument) is smaller than this value, then ANSYS FLUENT's time step is returned.

/* Particle time step control UDF for DPM  */

#include "udf.h"
#include "dpm.h"

DEFINE_DPM_TIMESTEP(limit_to_fifth_of_prt,p,dt)
{
   real drag_factor = 0.;
   real p_relax_time;
   cphase_state_t *c = &(p->cphase);

   /* compute particle relaxation time */
   if (P_DIAM(p) != 0.0)
     drag_factor = DragCoeff(p) * c->mu / ( P_RHO(p) * P_DIAM(p) * P_DIAM(p));
   else
     drag_factor = 1.;

   p_relax_time = 1./drag_factor;

   /* check the condition and return the time step */
   if (dt > p_relax_time/5.)
     {
       return p_relax_time/5.;
     }

   return dt;
}

After the UDF that you have defined using DEFINE_DPM_TIMESTEP is interpreted (Chapter 4) or compiled (Chapter 5), the name of the argument that you supplied as the first DEFINE macro argument will become visible and selectable for DPM Timestep in the Discrete Phase Model dialog box in ANSYS FLUENT. See Section 6.4.14 for details on how to hook your DEFINE_DPM_TIMESTEP UDF to ANSYS FLUENT.

2.5.14 DEFINE_DPM_TIMESTEP

2.5.14 `DEFINE_DPM_TIMESTEP`